It's fairl easy to take opponent mages out with your magic users alone:
Pierce Magic, Khelben's Warding Whip, Spel Thrust or Secret Word are really handy spells that will remove any of his magical defenses. This is very important the higher the level of your opponent, since they will have spell protections typically in a contingency. Once spell protections are removed use any spell to damage him. Breach will remove combat protections (stoneskin, prot. from magical weapons etc.), which is usefull if you want your fighters to get to him. Also always keep a non-magical weapon of your fighters specialisation to hit mages with prot. from magical weapons.
Magic missile is always good because it always hits the target, is cast almost instantly and thereby can interrupt spellcasting from enemies.
Other usefull spells:
Jaheira: Summon Fire Elemental (one of your partys absolute best spells!),
Insect Plague (interrupts spells, enemy loses morale)
Defensive Harmony
Call Lightning (deals out some serious damage)
Aerie: Holy Smythe (very powerfull, since most enemies in the game are evil)
Chant (cast it before a battle)
Protection from evil 10' (alwys good, especially when summoning Demons)
Conjure Air Elemental
Mage spells: Death Spell, Chaos, Fireball (as long as your fighters are not already closing in on enemies), Haste (more attacks), Slow, Dire Charm and many more.
Your mages should at least have Globe of Invulnerability and Stoneskin to keep enemy mages from interrupting spellcasting.
When you spot an enemy, let them come to you. That way you can fire some area spels without worrying about your party being hurt.
Always cast spells which affect your party BEFORE the battle, so you only have to concentrate on offensive spells during the battle...
Try to play through one time with only one (at most) fighter, and you'll see how powerfull you can REALLY get.....And remember to learn spells from scrolls rather than selling them, as you get xp for that.
Have fun